It is inherent in the electric power supply that power supply conforming to electricity consumption, what is called “simultaneous supply of equivalent electricity” is required. Meanwhile, developments of new energy resources, including power generation by wind and solar photovoltaic power generation, have been made increasingly in recent years. However, the power generation by wind, the solar photovoltaic power generation, and the like are all power sources which are irregular in output of power generation, due to which these power sources cannot serve as a satisfactory power supply source by themselves. Consideration is now being made on combination with some storage battery to stabilize the output of power generation. JP Laid-open (Unexamined) Patent Publication No. Sho 61-218070 describes the technique using a redox flow battery as the storage battery.
This publication describes that a lead storage battery, which does not need any moving parts, such as a pump and the like, is further annexed to the redox flow battery, in order to reduce a pump power loss and a loss caused by a shunt current, which contributes to reduction of combined efficiency of the redox flow battery, in response to input power.
In general, the redox flow battery is used for equalization of load or for countermeasure to voltage sag. FIG. 8 shows an explanatory view showing an operating principle of a general redox flow secondary battery. This battery has a cell 100 which is separated into a positive electrode cell 100A and a negative electrode cell 100B by a membrane 103 of an ion-exchange membrane. A positive electrode 104 and a negative electrode 105 are contained in the positive electrode cell 100A and the negative electrode cell 100B, respectively. A positive electrode tank 101 for feeding and discharging positive electrolytic solution to and from the positive electrode cell 100A is connected to the positive electrode cell 100A through conduit pipes 106, 107. Similarly, a negative electrode tank 102 for feeding and discharging negative electrolytic solution to and from the negative electrode cell 100B is connected to the negative electrode cell 100B through conduit pipes 109, 110. Aqueous solution containing ions that change in valence, such as vanadium ion, is used for the respective electrolytes and is circulated by using pumps 108, 111, to charge or discharge with an ionic valence change reaction at the positive and negative electrodes 104, 105. For example, when the electrolyte containing the vanadium ions is used, the following reactions occur in the cell during the charge or discharge of electricity:Positive electrode: V4+→V5++e−(Charge)V4+←V5++e−(Discharge)Negative electrode: V3++e−→V2+(Charge)V3++e−←V2+(Discharge)
However, the annex of the lead storage battery for the purpose of improving reduction of efficiency caused by weak power generation or load power at the time of electric charge or discharge or preventing increase of a system loss at the time of electric charge or discharge, as in the technique disclosed in the publication above, causes the problems of not only increase in production costs but also increase in scale of facilities.
Driving the pump for feeding and discharging the electrolyte to and from the cells is absolutely necessary for the redox flow battery. Due to this, the conventional employs the annex of the lead storage battery to the power source which is irregular in output of power generation, while suffering from the disadvantages of increase in production costs and others.
In general, a system loss means a total of a battery loss and a converter loss. The technique described in the publication cited above takes a pump power loss and a shunt current loss into consideration as factors of the battery loss, but takes no thought of an efficiency loss caused by a battery resistance (cell resistance). In view of this, the system that can provide further reduced loss is being desired.
Meanwhile, there are some conventional facilities in which only the redox flow battery is combined with the power source which is irregular in output of power generation, without the lead storage battery annexed thereto, taking no thought of the reduction of efficiency caused by weak generated power or load power at the time of electric charge or discharge. In these facilities, no study is made on how great magnitude the storage battery should be for combination with the magnitude (output of power generation, variation in output of power generation, etc.) of the power source which is irregular in output of power generation, such as power generation by wind and solar photovoltaic power generation, and no design guideline thereof is provided. In the circumstances above, the redox flow battery is practically operated, combining with the storage battery of a reasonable magnitude, or specifically, a magnitude of output about one half the total output of power generation, but this operation induces a great system loss, leading to deterioration of system efficiency.
Also, it is common that no study is made on how great the magnitude of the converter should be for combination with the magnitude (output of power generation, variation in output of power generation, etc.) of the power source which is irregular in output of power generation, such as power generation by wind and solar photovoltaic power generation, and no design guideline thereof is provided. For this, the redox flow battery combined with the power source irregular in output of power generation takes no thought of the converter loss.
Further, no study has been made on the design guideline that can provide further reduction of the system loss for the redox flow battery used for equalization of load power or countermeasure to irregular variation in power consumption, such as voltage sag, as well as for the redox flow battery used for stabilization of output of power source which is irregular in output of power generation.
Additionally, a method of designing an optimum redox flow battery considering not only the system loss but also reduction of production costs and scale of the facilities is being desired.
It is a primary object of the present invention to provide a method of designing a redox flow battery that can provide a more optimal operation for the redox flow battery which is irregularly operated for stabilization of an output of power generation of a power source which is unstable in generated power, as well as for control of supply and demand of electricity.
It is another object of the present invention to provide a method of designing a redox flow battery system that can reduce a system loss caused by weak generation power or load power at the time of electric charge or discharge, without using the lead storage battery.